ANDREW MANUEL CRESPO Probable Cause Pluralism abstract. The constitutionality of a search or seizure typically depends upon the connection between the target of that search or seizure and some allegation of illegal behavior—a connection assessed by asking whether the search or seizure is supported by probable cause. But as central as probable cause is to the Fourth Amendment’s administration, no one seems to know what it means or how it operates. Indeed, the Supreme Court insists it is “not possible” to define the term, hold- ing instead that the probable-cause inquiry entails no more than the application of “common sense” to “the totality of the circumstances.” Viewed charitably, this refusal to elaborate on the meaning of probable cause stems from an understandable desire for doctrinal flexibility in the face of weighty and competing law-enforcement demands. But the Court’s doctrinal approach is also routinely criticized as an “I know it when I see it” jurisprudence that is ill equipped to safeguard civil liberties in the numerous interactions between civilians and law-enforcement actors. This tension between doctrinal flexibility and structure is the animating dilemma of probable- cause jurisprudence—a dilemma that this Article attempts to navigate and, ultimately, to resolve. To do so, it urges a rejection of an often invoked—if not always followed—tenet of Supreme Court doctrine: probable cause unitarianism. That dominant idea, expressly endorsed in many of the Court’s leading precedents, holds that whatever probable cause means, it ought to entail the same basic analytic method and be judged by the same substantive standard, from one case to another. But on close inspection, the Supreme Court’s existing jurisprudence contains seeds of an alterna- tive—and superior—conception of probable cause, which this Article terms probable cause plural- ism. On this view, probable cause is an open-textured and capacious idea that can comfortably encompass distinct analytic frameworks and substantive standards, each of which can be tailored to the unique epistemological and normative challenges posed by different types of Fourth Amend- ment events. Probable-cause analysis can be statistically driven or intuitively assessed; it can de- mand compelling evidence of illegal behavior or only an occasionally satisfied profile; it can pre- sume the credibility of some types of witnesses while treating others with deserved skepticism or disbelief. It can, in short, come to mean something—if it gives up on meaning any one thing in all cases. In its current form, probable cause’s pluralism is nascent, implicit, and undertheorized—and is thus at best a stunted and haphazard collection of disparate ideas. This Article’s central contri- bution is to bring those ideas together, refining and synthesizing them into a comprehensive ac- count of what a pluralist theory of probable cause could and should look like. Specifically, by or- ganizing probable cause around three central analytic axes—which in turn ask how to assess evidentiary claims, how to assess proponents of such claims, and how to determine the certainty thresholds for those two assessments—this Article constructs a universally applicable framework for determining the constitutionality of any given search or seizure. With that framework in hand, scholars and jurists will be better equipped to reason through the many and varied cases to come and better able to assess the many cases that have come before. 1276 probable cause pluralism author. Professor of Law, Harvard Law School. I am grateful to many colleagues whose thoughtful comments helped to improve and refine this Article: Paulina Arnold, Monica Bell, Ra- bia Belt, Nikolas Bowie, Kevin Cole, Andrew Ferguson, John Goldberg, Andrew Hellman, Eliza- beth Kamali, Orin Kerr, Genevieve Lakier, Cynthia Lee, Benjamin Levin, Anna Lvovsky, John Manning, Justin Murray, Catherine Padhi, John Rappaport, Richard Re, Daphna Renan, Daniel Richman, Adam Samaha, Abigail Shafroth, Jocelyn Simonson, Adrienne Spiegel, Carol Steiker, Jordan Woods, Crystal Yang, and David Zionts. My thanks as well to the participants in the St. John’s University Law School Faculty Colloquium and the Harvard Law School Criminal Justice Workshop for helpful comments on prior drafts. I am also grateful to my diligent research assis- tants who helped me compile and analyze the cases in this Article’s appendix: Gavan Duffy Gideon, Riley Majeune-Fagan, Grace McLaughlin, Steven Palmer, and Parisa Sadeghi. This Article went to print as the U.S. Supreme Court was considering a potentially related case, Kansas v. Glover, 139 S. Ct. 1445 (2019). For an application of some of the ideas expressed here to that case, see Brief for Professor Andrew Manuel Crespo as Amicus Curiae in Support of Affirmance, Kansas v. Glover, 139 S. Ct. 1445 (2019) (No. 18-556), 2019 WL 4256217, reprinted in Andrew Manuel Crespo, The Unavoidably Empirical Fourth Amendment: A Case Study of Kansas v. Glover, 1 CTS. & JUST. L.J. 217 (2019). 1277 the yale law journal 129:1276 2020 article contents introduction 1279 i. axis one: assessing evidentiary claims 1288 A. Thin Scripts and the Statistical Method 1291 1. The Philosophical Critique 1294 2. The Practical Critique 1300 3. The Doctrinal Critique 1302 B. Narrative Mosaics and the Qualitative Method 1309 1. The Subjectivity Critique 1311 2. The Consistency Critique 1313 3. The Deference Critique 1314 C. Mixed(-Up) Claims 1318 1. Mixed Claims 1318 2. Mixed-Up Claims: Faux Thin Scripts 1319 3. Mixed-Up Claims: Thick Scripts 1320 ii. axis two: assessing claim-proponent credibility 1321 A. Civilians 1328 B. Officers 1330 C. Informants 1333 D. Anonyms 1336 iii. axis three: determining certainty thresholds 1340 A. The Anchor 1345 B. The Adjustments 1350 C. The Poles 1354 iv. pulling pluralism together 1357 A. Prospective Assessment: Deciding Future Cases 1359 B. Retrospective Assessment: Evaluating Existing Doctrine 1364 conclusion 1369 appendix 1371 1278 probable cause pluralism introduction Of the fifty-four words in the Fourth Amendment, the two that matter most are the least understood—probable cause. Doctrinally and conceptually, “probable cause lies at the heart of the [F]ourth [A]mendment” for one simple reason: the requirement to demonstrate probable cause—or its junior partner, reasonable suspicion—constitutes the core substantive constraint on police power in the United States.1 It is “the line of distinction” between legal and illegal searches and seizures.2 And yet, two centuries after the Supreme Court first applied the phrase, scholars continue to describe it as “elusive,” “hopelessly indeterminate,” and “shrouded in mystery.”3 Courts, meanwhile, suggest it might just be the most confusing “two-word term in American law.”4 The challenge, to be clear, is not figuring out the basic question that probable cause poses, for on that score there is general agreement: to satisfy the Fourth Amendment’s core substantive requirement, the government must point to facts 1. Albert W. Alschuler, Bright Line Fever and the Fourth Amendment, 45 U. PITT. L. REV. 227, 243 (1984); see also Dunaway v. New York, 442 U.S. 200, 213 (1979) (describing “[t]he central im- portance of the probable-cause requirement to the . Fourth Amendment’s guarantees”). For reasons detailed infra Section III.B, this Article generally uses the phrase “probable cause” to include “the pint-sized version of probable cause required for stop-and-frisk,” more com- monly known as “reasonable suspicion.” Anthony G. Amsterdam, Perspectives on the Fourth Amendment, 58 MINN. L. REV. 349, 414 (1974). 2. Carroll v. United States, 267 U.S. 132, 156 (1925). This Article focuses on the Fourth Amend- ment’s substance and thus brackets its main procedural line of distinction: the so-called war- rant requirement. Important as that procedural constraint may be, it tends to be honored in the breach. See RONALD J. ALLEN ET AL., CRIMINAL PROCEDURE: INVESTIGATIONS AND THE RIGHT TO COUNSEL 449 (3d ed. 2017); cf. infra note 231 (discussing the warrant requirement). Probable cause, however, is required whether a warrant is or is not. See U.S. CONST. amend. IV (“[N]o warrants shall issue, but upon probable cause . .”); Akhil Reed Amar, Fourth Amendment First Principles, 107 HARV. L. REV. 757, 782 (1994) (“[E]ven warrantless searches and seizures ordinarily must be backed by ‘probable cause.’”). “The emphasis on encouraging the use of warrants [thus] overlooks probable cause as the primary protection of the citizen’s privacy.” Joseph D. Grano, A Dilemma for Defense Counsel: Spinelli-Harris Search Warrants and the Possibility of Police Perjury, 1971 U. ILL. L.F. 405, 456; see also Josh Bowers, Probable Cause, Constitutional Reasonableness, and the Unrecognized Point of a “Pointless Indignity,” 66 STAN. L. REV. 987, 988 (2014) (“A police officer needs probable cause to arrest a suspect,” but “once he has it, he typically needs no more.”). 3. Craig S. Lerner, The Reasonableness of Probable Cause, 81 TEX. L. REV. 951, 953, 957 (2003); see also Christopher Slobogin, Let’s Not Bury Terry: A Call for Rejuvenation of the Proportionality Principle, 72 ST. JOHN’S L. REV. 1053, 1082 (1998) (“[P]robable cause . is the standard with which we are most familiar—except that we don’t really know what it means.”). 4. Holmes v. State, 796 A.2d 90, 98 (Md. 2002) (“With the possible exception of ‘due pro- cess’ . .” ). 1279 the yale law journal 129:1276 2020 that provide some basis to believe that “an offense has been or is being commit- ted” by